The use, by virtue of their capacity to undergo nanostructuring, of block copolymers in the fields of electronics or optoelectronics is now well known. This is illustrated in particular in the article by Cheng et al. (ACS nano, vol. 4, No. 8, 4815-4823, 2010). It is possible more particularly to structure the arrangement of the blocks constituting the copolymers on scales of smaller than 50 nm.
The desired structuring (for example, generation of domains perpendicular to the surface), however, requires the preparation of the substrate to which the block copolymer is applied, for the purpose of controlling the surface energy. Among the possibilities that are known, a polymer, more particularly a random polymer, is applied to the substrate, it being possible for the monomers of said copolymer to be wholly or partly identical with those used in the block copolymer it is desired to apply.
Moreover, if the wish is to prevent, for example, the diffusion of the polymer, it is preferable to graft and/or crosslink the polymer on the surface, through the use of appropriate chemical functionalities. Grafting means the formation of a bond—a covalent bond, for example—between the substrate and the polymer. Crosslinking means the presence of a plurality of bonds between the polymer chains.
When the same polymer is used sequentially, the grafting is designated as multiple. In fact, the grafting of the polymer is repeated several times (application, annealing and solvent rinsing). When polymers of different molecular mass and/or composition are applied, then the grafting is designated as hierarchical. In the latter case, the application of the polymers can be carried out simultaneously or successively.
Among the various possibilities used for orienting the morphology of a block copolymer on a surface, a layer of a random PMMA/PS polymer is applied beforehand to the surface.
Mansky et al. in Science, Vol. 275, pages 1458-1460 (7 Mar. 1997), showed that a random poly(methyl methacrylate-co-styrene) (PMMA/PS) polymer functionalized by a hydroxyl function at the chain end allows effective grafting of the polymer to the surface. The authors attribute the grafting capacity of these polymers to the presence of the terminal hydroxyl group originating from the initiator; this constitutes a condensation grafting mechanism, which is not very effective from the standpoint of the temperature and times that are required, typically 24 to 48 h at 140° C., in this publication.
At a certain molar fraction of the methyl methacrylate and styrene (MMA and STY) monomers, the interface energies of a random polymer with PS and PMMA, respectively, are strictly the same (Mansky et al., Macromolecules 1997, 30, 6810-6813). This situation arises in the case of a silicon support having a fine oxide layer on the surface. In this case, this may present a drawback, since the ideal composition of the random polymer must exhibit exactly this fraction in order for the interface energies with the PS and with the PMMA to be the same. When the composition of the random polymer changes, the authors showed that a PS-PMMA diblock copolymer applied to the random polymer may exhibit morphologies which are dependent on the composition of the random polymer. It is therefore possible to change the morphology of the diblock copolymer in the event of inconsistency of the MMA/STY fraction of the random polymer.
More recently, certain authors (Han et al., Macromolecules, 2008, 9090-9097, Ji et al., Macromolecules, 2008, 9098-9103, Insik In et al., Langmuir, 2006, 22, 7855-7860) have shown that it is possible, advantageously, to enhance the grafting of the random polymer on the surface by introducing—no longer at the chain end but within the random polymer itself—a plurality of functionalities such as hydroxyl or epoxy. In this case, the polymer is grafted by a plurality of functions on the surface (in the case of hydroxyl) and also crosslinked at the surface (in the case of epoxy).
Although the approaches previously described in the literature do allow certain controls over the orientation of a block copolymer on a surface treated with a random polymer, numerous defects of morphology of the block copolymer applied often arise, and are detrimental to industrial applications particularly in the electronics fields.
The applicant has now found that the hierarchical or multiple grafting of at least two polymers of equivalent or non-equivalent composition and of identical or different molecular mass on a surface produces an orientation of a subsequently deposited block copolymer with a minimum of defects, as compared with the grafting of a single layer of polymer. The applicant has observed that the thickness of the layers does not change in the course of the hierarchical or multiple grafting operations, thereby ensuring a consistency in the lithographic parameters until the block copolymer is applied.
Another advantage of the invention relates to the adjustment of the surface energy. When a single polymer is used, as described in the prior art, it corresponds to a given surface energy after deposition, allowing the subsequent deposition of a block copolymer with the same surface energy. However, it is not always easy to have precisely the same surface energy between the polymer used for preparing the surface and the block copolymer which is to be structured. With the present invention, it is possible to modify the surface energy of the first polymer applied to the surface, by means of a second polymer, with a different composition, thereby allowing fine-tuning of the surface energy and hence a better equalization of the surface energies between the polymers used to prepare the surface and the block copolymer which is to be structured. Si Nealey et al. (J. Vac. Sci. Technol. B 27(6), 2009) describe the application of two polymers in succession (a polystyrene then a PMMA), the aim of this application being to adjust the surface energies, without any consideration of the consequence with regard to structuring defects of a block copolymer subsequently deposited on the treated surface.
Similarly, T. Vu et al., in Macromolecules, 2011, 44 (15), pp. 6121-6127 consider the possibility of implementing a second grafting step, without consideration for any consequence for the structuring defects of a block copolymer subsequently deposited on the treated surface.